This invention relates generally to a method of and maskant composition for preventing the deposition of a coating on a predetermined portion of a substrate and, more particularly, to such a method and maskant which prevents the deposition of an aluminide-type coating on a nickel, cobalt or iron-based substrate by reacting with the aluminum containing vapors produced during the coating/deposition process thereby preventing deposition of the coating on the predetermined masked portion of the substrate.
The coating of metallic substrates is extremely common, particularly in the automotive and aerospace industries where it is desirable that a particular part possess numerous and varied physical properties. By coating or plating an alloy or other coating onto the surface of such a part, certain desirable surface physical properties, not otherwise characterstic to the substrate, may be achieved. For example, in the gas turbine engine industry, turbine blades and nozzle guide vanes are commonly fabricated from high temperature, nickel, cobalt and iron-based alloys since such alloys generally possess excellent strength characteristics at the elevated temperatures at which these blades and vanes are designed to operate. Such alloys are, however, susceptable to surface oxidation with its corresponding deleterious effects which may result in premature part failure. In order to minimize this problem, coatings such as aluminides are commonly applied. See, in this regard, U.S. Pat. Nos. 3,450,512 which issued to D. H. Maxwell on June 17, 1969; 3,436,249 which issued on Apr. 1, 1969 to G. Lambert et al.; and 3,544,348 which issued to D. H. Boone et al. on Dec. 1, 1970.
These coatings may be applied to the substrate in a variety of ways including, for example, pack cementation and hot spraying. With respect to either process, since only certain portions of the substrate are to be coated, it becomes necessary to mask off or otherwise prevent deposition of the coating on the areas not to be coated.
Historically, there have been two commonly accepted ways in which deposition can be prevented. The first is a complete physical block, normally accomplished by the use of a thick layer of an inert material. The second common way to prevent deposition is by the use of a sacrificial type maskant which will be sacrificed in favor of the substrate.
A third approach which, heretofore, has not been successfully employed due principally to the inability to control the reaction, constitutes the masking of the portion of the substrate to be protected with a reactive-type masking material capable of chemically reacting with the vapors produced during the coating process to thereby prevent deposition. Such a reactive type maskant, when fully controlled, offers numerous advantages over the physical block and sacrificial types. Among these advantages are the ability to use a thinner layer of maskant thus reducing cost; the ability to insure complete prevention of coating deposition in covered areas; and, when properly controlled, the ability to precisely control the line of demarcation between the coated and uncoated areas.
In the past, attempts at using reactive-type maskants have failed due principally to the inability to control the reaction between the maskant and the coating, resulting in it having been, heretofore, impossible to control the exact line of demarcation between the masked and unmasked areas.
Against the foregoing background, it is a primary objective of the present invention to provide a controllable, reactive-type maskant composition.
It is another objective of the present invention to provide a controllable, reactive type maskant composition which for use in preventing the deposition of aluminum based coatings on nickel, cobalt and iron-based alloys.
It is another object of the present invention to provide a method for preventing the deposition of a coating on a predetermined portion of a substrate using a reactive-type maskant composition.